[PATCH v2] linux-user/strace: Improve strace output for read() and getcwd()

[Helge Deller <deller@gmx.de>]

The strace functionality in qemu-user lacks the possibility to trace
which real values get returned to pointers in userspace by syscalls.

For example, the read() and getcwd() syscalls currently only show the
destination address where the syscalls should put the return values:
2532 read(3,0xff80038c,512) = 512
2532 getcwd(0x18180,4096) = 9

With the patch below, one now can specify in print_syscall_late() which
syscalls should be executed first, before they get printed.
After adding the read() and getcwd() syscalls, we now get this output in
with strace instead:
1708 read(3,"\177ELF\1\2\1\3\0\0\0\0\0\0\0\0\0\3\0\17\0\0\0\1\0\2bl\0\0\04"...,512) = 512
1708 getcwd("/usr/bin",4096) = 9

This patch adds just the framework with the respective implemenations for
read() and getcwd(). If applied, more functions can be added easily later.

Signed-off-by: Helge Deller <deller@gmx.de>
---
Changes v2 to v1:
- Print syscall name, e.g. "read(" before calling the syscall. In case of
  crash people will then be able to see that it crashed in this "late" call.
  Based on feedback by Aleksandar Markovic <aleksandar.m.mail@gmail.com>


diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index f6f5fe5fbb..65557660a9 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -371,6 +371,10 @@ extern long safe_syscall_base(int *pending, long number, ...);
 int host_to_target_waitstatus(int status);

 /* strace.c */
+const struct syscallname *print_syscall_prologue_num(int num);
+void print_syscall_late(const struct syscallname *sc,
+              abi_long arg1, abi_long arg2, abi_long arg3,
+              abi_long arg4, abi_long arg5, abi_long arg6);
 void print_syscall(int num,
                    abi_long arg1, abi_long arg2, abi_long arg3,
                    abi_long arg4, abi_long arg5, abi_long arg6);
diff --git a/linux-user/strace.c b/linux-user/strace.c
index de43238fa4..dc963accd5 100644
--- a/linux-user/strace.c
+++ b/linux-user/strace.c
@@ -61,6 +61,7 @@ UNUSED static void print_open_flags(abi_long, int);
 UNUSED static void print_syscall_prologue(const struct syscallname *);
 UNUSED static void print_syscall_epilogue(const struct syscallname *);
 UNUSED static void print_string(abi_long, int);
+UNUSED static void print_encoded_string(abi_long addr, unsigned int maxlen, int last);
 UNUSED static void print_buf(abi_long addr, abi_long len, int last);
 UNUSED static void print_raw_param(const char *, abi_long, int);
 UNUSED static void print_timeval(abi_ulong, int);
@@ -1196,6 +1197,26 @@ print_syscall_prologue(const struct syscallname *sc)
     gemu_log("%s(", sc->name);
 }

+void
+print_syscall_late(const struct syscallname *sc,
+              abi_long arg1, abi_long arg2, abi_long arg3,
+              abi_long arg4, abi_long arg5, abi_long arg6)
+{
+    const char *format = TARGET_ABI_FMT_ld "," TARGET_ABI_FMT_ld ","
+                    TARGET_ABI_FMT_ld "," TARGET_ABI_FMT_ld ","
+                    TARGET_ABI_FMT_ld "," TARGET_ABI_FMT_ld ")";
+
+    if (sc->call != NULL) {
+        sc->call(sc, arg1, arg2, arg3, arg4, arg5, arg6);
+    } else {
+        if (sc->format != NULL) {
+            format = sc->format;
+        }
+        gemu_log(format, arg1, arg2, arg3, arg4, arg5, arg6);
+    }
+}
+
+
 /*ARGSUSED*/
 static void
 print_syscall_epilogue(const struct syscallname *sc)
@@ -1204,6 +1225,37 @@ print_syscall_epilogue(const struct syscallname *sc)
     gemu_log(")");
 }

+#define MAX_ENCODED_CHARS 32
+static void
+print_encoded_string(abi_long addr, unsigned int maxlen, int last)
+{
+    unsigned int maxout;
+    char *s, *str;
+
+    s = lock_user_string(addr);
+    if (s == NULL) {
+        /* can't get string out of it, so print it as pointer */
+        print_pointer(addr, last);
+        return;
+    }
+
+    str = s;
+    gemu_log("\"");
+    maxout = MIN(maxlen, MAX_ENCODED_CHARS);
+    while (maxout--) {
+        unsigned char c = *str++;
+        if (isprint(c)) {
+            gemu_log("%c", c);
+        } else {
+            gemu_log("\\%o", (unsigned int) c);
+        }
+    }
+    unlock_user(s, addr, 0);
+
+    gemu_log("\"%s%s", maxlen > MAX_ENCODED_CHARS ? "..." : "",
+                    get_comma(last));
+}
+
 static void
 print_string(abi_long addr, int last)
 {
@@ -1633,6 +1685,18 @@ print_futimesat(const struct syscallname *name,
 }
 #endif

+#ifdef TARGET_NR_getcwd
+static void
+print_getcwd(const struct syscallname *name,
+    abi_long arg0, abi_long arg1, abi_long arg2,
+    abi_long arg3, abi_long arg4, abi_long arg5)
+{
+    print_string(arg0, 0);
+    print_raw_param("%u", arg1, 1);
+    print_syscall_epilogue(name);
+}
+#endif
+
 #ifdef TARGET_NR_settimeofday
 static void
 print_settimeofday(const struct syscallname *name,
@@ -2428,6 +2492,19 @@ print_fstatat64(const struct syscallname *name,
 #define print_newfstatat    print_fstatat64
 #endif

+#ifdef TARGET_NR_read
+static void
+print_read(const struct syscallname *name,
+    abi_long arg0, abi_long arg1, abi_long arg2,
+    abi_long arg3, abi_long arg4, abi_long arg5)
+{
+    print_raw_param("%d", arg0, 0);
+    print_encoded_string(arg1, arg2, 0);
+    print_raw_param("%u", arg2, 1);
+    print_syscall_epilogue(name);
+}
+#endif
+
 #ifdef TARGET_NR_readlink
 static void
 print_readlink(const struct syscallname *name,
@@ -2817,6 +2894,22 @@ static int nsyscalls = ARRAY_SIZE(scnames);
 /*
  * The public interface to this module.
  */
+
+const struct syscallname *print_syscall_prologue_num(int num)
+{
+    int i;
+
+    gemu_log("%d ", getpid());
+
+    for (i = 0; i < nsyscalls; i++)
+        if (scnames[i].nr == num) {
+            print_syscall_prologue(&scnames[i]);
+            return &scnames[i];
+        }
+    gemu_log("Unknown syscall %d\n", num);
+    return NULL; /* will crash qemu */
+}
+
 void
 print_syscall(int num,
               abi_long arg1, abi_long arg2, abi_long arg3,
diff --git a/linux-user/strace.list b/linux-user/strace.list
index d49a1e92a8..220b1f4c46 100644
--- a/linux-user/strace.list
+++ b/linux-user/strace.list
@@ -272,7 +272,7 @@
 { TARGET_NR_getcpu, "getcpu" , "%s(%p,%d)", NULL, NULL },
 #endif
 #ifdef TARGET_NR_getcwd
-{ TARGET_NR_getcwd, "getcwd" , "%s(%p,%d)", NULL, NULL },
+{ TARGET_NR_getcwd, "getcwd" , NULL, print_getcwd, NULL },
 #endif
 #ifdef TARGET_NR_getdents
 { TARGET_NR_getdents, "getdents" , NULL, NULL, NULL },
@@ -1080,7 +1080,7 @@
 { TARGET_NR_quotactl, "quotactl" , NULL, NULL, NULL },
 #endif
 #ifdef TARGET_NR_read
-{ TARGET_NR_read, "read" , "%s(%d,%#x,%d)", NULL, NULL },
+{ TARGET_NR_read, "read" , NULL, print_read, NULL },
 #endif
 #ifdef TARGET_NR_readahead
 { TARGET_NR_readahead, "readahead" , NULL, NULL, NULL },
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index ce399a55f0..7207826137 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -12069,6 +12069,21 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
     return ret;
 }

+/*
+ * True if this syscall should be printed after having called the native
+ * syscall, so that values which are fed back to userspace gets printed.
+ */
+static int is_print_syscall_late(int syscall)
+{
+    switch (syscall) {
+    case TARGET_NR_getcwd:
+    case TARGET_NR_read:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
 abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
                     abi_long arg2, abi_long arg3, abi_long arg4,
                     abi_long arg5, abi_long arg6, abi_long arg7,
@@ -12095,9 +12110,20 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
                          arg2, arg3, arg4, arg5, arg6, arg7, arg8);

     if (unlikely(do_strace)) {
-        print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
+        const struct syscallname *sc;
+        int late_printing;
+
+        late_printing = is_print_syscall_late(num);
+        if (late_printing) {
+            sc = print_syscall_prologue_num(num);
+        } else {
+            print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
+        }
         ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
                           arg5, arg6, arg7, arg8);
+        if (late_printing) {
+            print_syscall_late(sc, arg1, arg2, arg3, arg4, arg5, arg6);
+        }
         print_syscall_ret(num, ret);
     } else {
         ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,